debug(mir): add comprehensive receiver tracing and block overwrite protection

This commit investigates ValueId(21) undefined error in Stage-B compilation.

Changes:
- src/mir/builder/builder_calls.rs:
  - Add NYASH_DEBUG_PARAM_RECEIVER=1 trace for method call receivers
  - Track variable_map lookups and ValueId mismatches
  - Log receiver origin and current_block context

- src/mir/builder/utils.rs:
  - Fix start_new_block() to avoid overwriting existing blocks
  - Check if block exists before calling add_block()
  - Prevents Copy instructions from being lost

- src/mir/function.rs:
  - Add warning log when replacing existing block
  - Helps detect block overwrite issues

- lang/src/mir/builder/ (Hako files):
  - Add debug prints for method lowering paths
  - These were not used (Stage-B uses Rust MIR Builder)
  - Kept for future Hako MIR Builder debugging

Key Discovery:
- Stage-B compilation uses Rust MIR Builder, not Hako MIR Builder
- ValueId(21) is undefined receiver in StageBArgsBox.resolve_src/1
- MIR shows: call_method ParserBox.length() [recv: %21] but %21 has no definition
- Likely caused by LocalSSA Copy emission failure or block overwrite

Next: Fix LocalSSA to ensure receiver Copy is properly emitted and preserved

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lang/src/mir/builder/func_body/basic_lower_box.hako

@@ -37,8 +37,12 @@ static box FuncBodyBasicLowerBox {
 
     // 2) Try simple Return(Method ...) pattern（params ベースの receiver のみ）
     {
+      print("[builder/funcs:return_method] trying func=" + func_name + " box=" + box_name)
       local mret = me._try_lower_return_method(func_name, box_name, params_arr, s)
-      if mret != null { return mret }
+      if mret != null {
+        print("[builder/funcs:return_method] SUCCESS via _try_lower_return_method")
+        return mret
+      }
     }
 
     // 3) Non-Loop processing: Local/If/Return パターンのみ
@@ -48,7 +52,14 @@ static box FuncBodyBasicLowerBox {
     s = me._inline_local_ints(s)
 
     // Try minimal lowers (same order as MirBuilderMinBox, restricted set)
-    { local out = LowerReturnMethodArrayMapBox.try_lower(s); if out != null { return me._rebind(out, func_name, box_name, params_arr, "[funcs/basic:return.method.arraymap]") } }
+    {
+      print("[builder/funcs] trying LowerReturnMethodArrayMapBox (LEGACY, const 0 receiver)")
+      local out = LowerReturnMethodArrayMapBox.try_lower(s)
+      if out != null {
+        print("[builder/funcs] SUCCESS via LowerReturnMethodArrayMapBox (WARNING: uses const 0 receiver!)")
+        return me._rebind(out, func_name, box_name, params_arr, "[funcs/basic:return.method.arraymap]")
+      }
+    }
     { local out = LowerReturnBinOpVarIntBox.try_lower(s); if out != null { return me._rebind(out, func_name, box_name, params_arr, "[funcs/basic:return.binop.varint]") } }
     { local out = LowerReturnBinOpVarVarBox.try_lower(s); if out != null { return me._rebind(out, func_name, box_name, params_arr, "[funcs/basic:return.binop.varvar]") } }
     { local out = LowerReturnBinOpBox.try_lower(s); if out != null { return me._rebind(out, func_name, box_name, params_arr, "[funcs/basic:return.binop.intint]") } }

lang/src/mir/builder/internal/lower_return_method_array_map_box.hako

@@ -12,16 +12,33 @@ static box LowerReturnMethodArrayMapBox {
     local k_m = JsonFragBox.index_of_from(s, "\"type\":\"Method\"", k_ret); if k_m < 0 { return null }
     // receiver must be Var(name)
     local k_recv = JsonFragBox.index_of_from(s, "\"recv\":{\"type\":\"Var\"", k_m); if k_recv < 0 { return null }
+
+    // Extract receiver variable name
+    local recv_name = null
+    {
+      local kn = JsonFragBox.index_of_from(s, "\"name\":\"", k_recv)
+      if kn >= 0 {
+        recv_name = JsonFragBox.read_string_after(s, kn + 8)
+      }
+    }
+
     local method = null
     { local km = JsonFragBox.index_of_from(s, "\"method\":\"", k_m); if km < 0 { return null } method = JsonFragBox.read_string_after(s, km + 9) }
     // Standardize: generate 'size' (len/length are accepted aliases at receiver)
     method = MethodAliasPolicy.normalize_size(method)
     // Allow basic methods
     if !(method == "size" || method == "length" || method == "len" || method == "get" || method == "set" || method == "push") { return null }
+
+    // Dev trace: show what we matched (always print for now)
+    print("[builder/arraymap] matched return method pattern")
+    print("[builder/arraymap]   method=" + method)
+    print("[builder/arraymap]   recv_name=" + recv_name)
+
     // Parse up to two Int args with actual values
     // Prepare instruction JSON text (avoid Box allocations in VM)
     local insts = ""
-    // Receiver placeholder: const 0 -> r1（Var 解決は未実装なので 0 を受信者に置く）
+    // FIXME: Receiver placeholder: const 0 -> r1（Var 解決は未実装なので 0 を受信者に置く）
+    // TODO: Use recv_name to find actual parameter slot instead of hardcoded 0
     insts = insts + "{\\\"op\\\":\\\"const\\\",\\\"dst\\\":1,\\\"value\\\":{\\\"type\\\":\\\"i64\\\",\\\"value\\\":0}}"
     local k_args = JsonFragBox.index_of_from(s, "\"args\":", k_m)
     local next_id = 2